JP4910470B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus that forms an image on a sheet.

2. Description of the Related Art Conventionally, an image forming apparatus that includes a sheet conveying unit that conveys a sheet and an image forming unit that forms an image on the conveyed sheet has been widely used as a printer or a copying machine. This image forming apparatus aligns the position in the width direction, which is perpendicular to the conveyance direction of the sheet conveyed by the sheet conveyance unit, with the position where an image is formed in the image formation unit. Images are formed at various positions. For example, when the position of the sheet is shifted in the width direction during the conveyance, an image is formed at a position shifted from the target position on the sheet. Here, in order to suppress the displacement of the image position with respect to the paper, for example, a sensor for detecting the position in the width direction of the paper to be conveyed is provided, and the image is statically placed on the image carrier according to the position of the paper detected by this sensor. An image forming apparatus that adjusts a position where an electrostatic latent image is formed is known (for example, see Patent Document 1).
JP 2000-33010 A

  However, in the above image forming apparatus, since the position is adjusted at the stage where the electrostatic latent image is formed on the image carrier, the electrostatic latent image is formed after the formation of the electrostatic latent image is started. Even if the position of the sheet is deviated before the toner image is developed and transferred to the sheet, this deviation is not reflected in the adjustment. Also, if the electrostatic latent image or toner image formation start on the image carrier is delayed and the formation position of the electrostatic latent image is adjusted after the paper is transported to the final position immediately before the toner image transfer. Since the electrostatic latent image is formed and the toner image is transferred, the sheet is kept on standby and the productivity of the image forming apparatus is lowered.

  In view of the above circumstances, an object of the present invention is to provide an image forming apparatus that improves the alignment performance between a sheet and an image while suppressing a decrease in productivity.

An image forming apparatus of the present invention that achieves the above object has an image carrier that carries an image on a surface and circulates, and exposes the surface of the image carrier in a direction perpendicular to the moving direction of the image carrier. To form an electrostatic latent image on the surface of the image carrier, develop the electrostatic latent image with toner to obtain a toner image, and finally transfer the toner image onto the conveyed paper. And an image forming unit that forms an image composed of a fixed toner image on the sheet by fixing, and
A paper transport unit for transporting paper on a paper transport path passing through the image forming unit,
The image forming unit adjusts the image forming position in the width direction by adjusting the latent image writing position in the width direction perpendicular to the paper transport direction on the surface of the image carrier.
The paper transport unit is disposed upstream of the image forming unit in the paper transport direction, and after the scanning in the image forming unit is started, the paper is moved in the width direction of the paper to move the width of the paper. A sheet shift unit for adjusting the position in the direction is provided.

  In the image forming apparatus of the present invention, the image forming unit adjusts the image forming position by adjusting the latent image writing position by scanning on the surface of the image carrier, and after the start of scanning in the image forming unit, the paper shift Adjusts the paper position. Accordingly, since the positional deviation of the paper that occurs after the start of scanning by the image forming unit is adjusted, the alignment performance between the paper and the image is improved. In addition, since it is possible to start scanning in the image forming unit without waiting for the sheet to be conveyed to a position immediately before the toner image transfer, it is possible to suppress a decrease in productivity of the image forming apparatus. Further, when the sheet is moved in a direction perpendicular to the conveyance direction, the sheet may be wrinkled or the sheet may be skewed. However, the image forming apparatus of the present invention starts forming an electrostatic latent image by the image forming unit. The shift of the sheet up to this time can be dealt with by adjusting the latent image writing position, and the movement of the sheet by the sheet shift unit can correspond to the fine adjustment of the shift occurring after the start of the formation of the electrostatic latent image. For this reason, the stress which arises in a paper by movement is suppressed.

Here, the image forming apparatus of the present invention described above is
A first sensor for measuring a position in the width direction of a sheet conveyed toward the sheet shift unit, which is disposed on the upstream side in the conveyance direction from the sheet shift unit;
A second sensor that is provided between the paper shift unit and the image forming unit and that measures the position in the width direction of the paper conveyed toward the image forming unit;
The paper shift unit corrects paper skew,
The image forming unit adjusts the image forming position in the width direction of the image formed on the paper based on the paper position measured by the first sensor;
The paper shift unit moves the paper in the width direction based on a difference between the paper position measured by the first sensor and the paper position measured by the second sensor. It is preferable that

  The position in the width direction of the sheet is likely to be shifted when the skew of the sheet, that is, the skew state is corrected by the sheet shift unit. Here, since the paper shift unit moves the paper based on the difference between the paper position of the first sensor and the paper position of the second sensor, the deviation of the paper position caused by the skew correction is corrected. The alignment performance between the paper and the image is improved.

  Further, in the image forming apparatus of the present invention, the paper shift unit includes a pair of rolls that contact each other to form a nip region, and the paper is abutted against the nip region to correct the skew of the paper. A registration roll is preferred.

  By configuring the shift unit with a pair of rolls, the function of correcting the skew of the sheet and the function of moving the sheet in the width direction can be realized with a simple configuration.

  In the image forming apparatus of the present invention, the image forming unit adjusts the image forming position in the width direction of the image formed on the paper based on the position of the paper measured by the first sensor. A first mode in which the sheet shift unit moves the sheet in the width direction of the sheet based on a difference in sheet position measured by the first sensor and the second sensor; and the sheet shift unit The movement of the sheet is stopped, and the image forming unit adjusts the image forming position in the width direction of the image formed on the sheet based on the position of the sheet measured by the second sensor. It is preferable to include a first switching unit that switches between modes.

  In the second mode, the paper position is measured downstream of the paper shift unit in the transport direction, and the latent image writing position is adjusted based on the measurement result. In this case, although there is a time for the paper to wait until the image is formed on the paper after the start of scanning, since the paper does not move in the width direction, the stress generated on the paper is further reduced. Further, since the positioning accuracy by adjusting the latent image writing position is higher than the adjustment accuracy by moving the sheet itself, the alignment performance between the sheet and the image is further improved. By switching between the second mode and the first mode, it is possible to select an appropriate alignment performance according to the required degree of productivity.

  In the image forming apparatus of the present invention, the first switching unit obtains paper thickness information indicating the thickness of the paper, and based on the paper thickness information, the first mode and the first mode are obtained. It is preferable to switch between the two modes.

  In the case of thick paper, since the time required for transferring the toner image is longer than that of normal paper, even if correction in the second mode is performed, the influence on productivity is small. By switching the mode based on the paper thickness information, it is possible to select appropriate alignment performance and productivity according to the paper thickness.

  In the image forming apparatus of the present invention, the image forming unit adjusts the image forming position in the width direction of the image formed on the paper based on the position of the paper measured by the first sensor. A first mode in which the sheet shift unit moves the sheet in the width direction of the sheet based on a difference in sheet position measured by the first sensor and the second sensor; and the sheet shift unit The movement of the sheet is stopped, and the image forming unit adjusts the image forming position in the width direction of the image formed on the sheet based on the position of the sheet measured by the first sensor. It is preferable to include a second switching unit that switches between and executes the mode.

  In the third mode, since the movement of the sheet by the sheet shift unit is stopped, image formation can be performed at a higher speed. Therefore, it is possible to select an appropriate alignment performance according to the required productivity.

Further, in the image forming apparatus of the present invention, the second sensor is a line sensor arranged at a position away from the center in the width direction on the conveyance path.
It is preferable that a friction compensation member having a friction coefficient equal to the friction coefficient of the line sensor is disposed at a position symmetrical to the line sensor with respect to the center in the width direction on the conveyance path.

  The second sensor detects the position of the edge of the paper by being arranged at a position away from the center in the width direction on the conveyance path. However, when the paper comes into contact with the line sensor, the second sensor may be skewed by frictional force. is there. Here, the friction compensation member having the same friction coefficient as that of the line sensor is disposed at a position symmetrical to the line sensor, so that the possibility that the sheet receives the frictional force evenly and skews is reduced. In this case, it is preferable that the sheet running surface of the friction compensation member has a shape symmetrical to the line sensor with respect to the center in the width direction on the transport path.

  As described above, according to the present invention, a decrease in productivity of the image forming apparatus is suppressed, and the alignment performance between the paper and the image is improved.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is an external view of an image forming apparatus according to an embodiment of the present invention.

  The image forming apparatus 1 shown in FIG. 1 includes an image forming mechanism 10, a paper transport mechanism 20, a control unit 30 that controls the operation of the entire image forming apparatus 1, and a paper tray 40 (40a, 40b, 40) that stores paper. 40c, 40d) and a touch panel 50 for inputting various information by the operation of the operator.

  The image forming mechanism 10 includes a scanner 11, a photosensitive drum 12 as an image carrier, a charger 13 that uniformly charges the photosensitive drum 12, a developing unit 14 that develops an electrostatic latent image into a toner image, and a toner image. A transfer unit 15 for transferring to a sheet, a fixing unit 16 for fixing toner on the sheet, and a cleaner 17 for collecting residual toner that has not been transferred are provided, and an image is formed on the sheet by electrophotography. An image signal transmitted from the outside is converted into a signal by exposure light in a laser output unit 11a included in the scanner 11, and rotates through a polygon mirror 11b, an fθ lens 11c, and a reflection lens 11d. An electrostatic latent image based on the image signal is formed on the drum 12. The electrostatic latent image is formed by the scanner 11 scanning the surface of the photosensitive drum 12 with exposure light in the direction of the rotation axis of the photosensitive drum 12. The electrostatic latent image is developed by the developing unit 14 to obtain a toner image. The toner is transferred by the transfer unit 15 to the sheet conveyed through the sheet conveyance path R. Thereafter, the residual toner is removed from the photosensitive drum 12 by the cleaner 17, and the photosensitive drum 12 is charged by the charger 13. In this manner, the photosensitive drum 12 carries an image on the surface and circulates. On the other hand, the paper on which the toner has been transferred by the transfer unit 15 is pressurized and heated by the fixing unit 16, and the toner image is fixed on the paper.

  Since the scanning direction of the scanner 11 coincides with the width direction of the sheet perpendicular to the sheet conveyance direction, the position in the width direction of the image formed by the image forming mechanism 10 is determined by the scanner 11 with respect to the exposure light. Adjustment is made free by controlling the scanning timing.

  The paper transport mechanism 20 includes pick-up rolls 21a to 21d for picking up the papers stored in the paper trays 40a to 40d, a picking roll 22a to 22d for picking up the picked up paper, a skew removing unit 25 for removing the skew of the paper, A transport belt 26 that transports the transfer unit 15 to the fixing unit 16 and discharge rolls 27a and 27b that discharge the paper are provided. The sheets accommodated in the sheet trays 40a to 40d pass through the image forming mechanism 10 by the pickup rolls 21a to 21d, the separating rolls 22a to 22d, the skew removing unit 25, the transport belt 26, and the discharge rolls 27a and 27b. It is transported on the transport route R. Further, the paper transport mechanism 20 includes a reverse transport unit 28 that reverses and transports the paper on which the image is formed, and a refeed unit that transports the reverse transported paper to the skew removing unit 25 again for duplex printing. 29 is also provided.

  The skew removing unit 25 includes guide members 251, 252, 253, and 254 that guide the paper along the paper transport path R, a transport roll 255 that feeds the paper between the guide members 251, 252, and 253, and a paper transfer unit. 15 is provided with a registration roll (hereinafter also referred to as a registration roll) 256. Unlike the other guide members 252, 253, and 254, the guide member 251 is a movable member that is fixed only on one side with respect to the paper transport path R, and is directed toward another guide member 253 by an elastic member (not shown). It is energized. The registration roll 256 is disposed upstream of the image forming mechanism 10 in the sheet conveyance direction, and also functions as a sheet shift unit that shifts the sheet in the width direction.

  A first sensor and a second sensor for measuring the position of a side edge, which is a side edge parallel to the paper transport direction, on the upstream side in the transport direction and the downstream side in the transport direction from the registration roll 256 on the paper transport path R. A sensor is arranged. Hereinafter, the side edge position of the sheet will be described as a position perpendicular to the sheet conveyance direction. Note that members other than the skew removing unit 25 are also provided with members for guiding the paper along the paper transport path R. However, these members are not illustrated.

  The control unit 30 receives signals from various sensors including the first sensor 257 and the second sensor 258 and controls the operation of each mechanism of the image forming apparatus 1. Although not shown, the control unit 30 controls the overall operation of the image forming system based on a program, a ROM that stores programs and tables, and a temporary storage area in the CPU. It has a RAM to be provided and an interface circuit that relays signals between the outside of the control unit 30 and the CPU. Various functions provided by the control unit 30 are realized based on programs executed by the CPU. The function of the control unit 30 will be described later.

  FIG. 2 is a diagram illustrating a schematic configuration of the skew removing unit illustrated in FIG. 1 as viewed from above the image forming apparatus. Each of the first sensor 257 and the second sensor 258 is a CCD (Charge Couple Device) line sensor in which a plurality of light receiving elements are arranged in a line and covered with polycarbonate resin, and is centered in the width direction on the transport path. At a position away from C, the light receiving elements are arranged so that the arrangement direction of the light receiving elements is perpendicular to the paper transport direction Y. A guide member 252 is disposed at a position facing the first sensor 257 across the path for transporting the paper P, and a guide member 254 is disposed at a position facing the second sensor 258. Has been. Both guide members 252 and 254 are made of black ABS resin. Here, the gap between the first sensor 257 and the second sensor 258 and the guide member 252 and the guide member 254 allows the sheet to pass freely through the gap while maintaining good sensor sensitivity. It is preferably kept in the range of about 0.5 mm to 1.5 mm so as not to be disturbed. The first sensor 257 and the second sensor 258 output an electrical signal indicating whether each light receiving element faces the paper or whether the paper does not exist and faces the guide members 252 and 254. Thereby, the side edge position of the paper is detected. Details of the output signal will be described later. A friction compensation member 259 is disposed at a position symmetrical to the second line sensor with respect to the center C in the width direction on the conveyance path. The friction compensation member 259 is made of the same polycarbonate resin as the second sensor 258 and has a friction coefficient equal to that of the line sensor. Further, the friction compensation member 259 has a sheet traveling surface, which is a surface facing the sheet to be conveyed, having a shape symmetrical to the second line sensor with respect to the center C in the width direction. For this reason, the sheet passing through the position of the second sensor 258 comes into contact with both the second sensor 258 and the friction compensation member 259 disposed at positions symmetrical to the center C in the width direction. Receive. Therefore, the skew of the paper after passing through the registration roll 256 is prevented. Note that the material of the guide members 252 and 254 may be, for example, a metal subjected to black nickel plating in addition to the resin. In addition, the first sensor 257 and the second sensor 258 may be configured by a photosensor other than a CCD, and the surface may be covered with an ABS resin instead of a polycarbonate resin. The friction compensation member 259 is preferably made of the same material as the surface material of the second sensor 258.

  The registration roll 256 is driven by a rotation drive motor 256R and a shift motor 256S. The rotation drive motor 256R conveys the sheet in the conveyance direction Y by rotating the registration roll 256. The shift motor 256S moves the sheet in the width direction X by moving the registration roll itself in the rotation axis direction. As described above, the function of correcting the skew of the sheet and the function of moving the sheet in the width direction are realized by a simple configuration using a pair of rolls.

  In the skew removing unit 25 shown in FIG. 2, the sheet conveyed by the conveying roll 255 (see FIG. 1) is guided by the guide members 251 and 252, passes over the first sensor 257, and is registered by the registration roll. Abutting 256 nip areas. The first sensor 257 detects the position of the side edge of the paper coming toward the registration roll 256. The registration roll 256 stops rotating when the sheet is abutted against it, but starts rotating in synchronization with the timing when the electrostatic latent image is formed on the photosensitive drum 12 (see FIG. 1). The paper is transported in the transport direction Y. The sheet passes through the position of the second sensor 258 and is conveyed toward the transfer unit 15. The second sensor 258 detects the side edge position of the sheet conveyed from the registration roll 256 toward the transfer unit 15.

  FIG. 3 is a diagram schematically illustrating how the paper of the skew removal unit is conveyed.

  As shown in part (a) of FIG. 3, the paper transported by the transport roll 255 is guided by the guide members 251 and 252 and proceeds in the direction of the registration roll 256 along the transport path. At this time, the registration roll 256 has stopped rotating. As shown in part (b) of FIG. 3, after the sheet is abutted against the registration roll 256, when the conveyance by the conveyance roll 255 is continued, between the conveyance roll 255 and the registration roll 256 of the sheet. And the guide member 251 is pushed outward. This increased paper curvature is called a loop. The paper is abutted against the registration roll 256 and a loop is formed, whereby the edge of the front edge of the paper is aligned with the registration roll 256, and the skew of the paper is corrected. When the skew is corrected, the paper moves in the width direction. That is, the side edge position of the paper after passing through the registration roll 256 is deviated from the side edge position before being abutted against the registration roll 256. In the image forming apparatus 1 of the present embodiment, the position of the side edge of the paper after passing through the registration roll 256 is detected by the second sensor 258 (see FIG. 2), and the registration roll 256 moves the paper in the width direction. Thus, the position of the side edge of the sheet is returned to the position when the first sensor 257 detects it.

  FIG. 4 is a functional block diagram of the image forming apparatus shown in FIG.

  The control unit 30 includes a scanning control unit 31, a mechanism control unit 32, a conveyance control unit 33, a side shift control unit 34, an image position adjustment unit 35, and a mode switching unit 36. A personal computer (PC) 60 is externally connected to the control unit 30.

  The image position adjustment unit 35 reads the side edge position of the sheet conveyed toward the registration roll 256 from the first sensor 257, and adjusts the latent image writing position based on the side edge position. The latent image writing position is obtained by adding a predetermined margin width to the value of the side edge position of the sheet.

  The scanning control unit 31 controls the operation of the laser output unit 11 a of the scanner 11 according to the image signal transmitted from the personal computer 60. At this time, the scanning control unit 31 controls the output timing of the laser output unit 11a based on the latent image forming position adjusted by the image position adjusting unit 35, whereby the exposure light corresponding to the image signal by the scanner 11 is controlled. By changing the scanning timing, an electrostatic latent image is formed at the adjusted latent image forming position.

  The mechanism control unit 32 controls the operations of the units 12 to 17 of the image forming mechanism 10 other than the scanner 11 to form an image on a sheet.

  The side shift control unit 34 controls the driving of the shift motor 256 </ b> S based on the difference between the paper position measured by the first sensor 257 and the paper position measured by the second sensor 258. As a result, the registration roll 256 moves the paper in the width direction. Specifically, the side shift control unit 34 drives the shift motor 256 </ b> S so that only the difference between the paper position measured by the first sensor 257 and the paper position measured by the second sensor 258 is obtained. The registration roll 256 is moved in the width direction. As a result, the position of the sheet is returned to the position of the sheet when measured by the first sensor 257.

  The conveyance control unit 33 controls the operation of the paper conveyance mechanism 20 other than the shift motor 256S to convey the paper.

  For example, the mode switching unit 36 obtains information on the print mode such as the paper type and the high-speed mode transmitted from the personal computer 60 along with the image signal, and in the paper trays 40a to 40d according to the information. The paper tray for the paper to be transported to the paper transport mechanism 20 is selected. Further, the mode switching unit 36 switches the adjustment mode of the paper position and the image position between the normal mode, the high speed mode, and the thick paper mode in accordance with the thickness information in the paper type information and the print mode information. The mode switching unit 36 can also perform mode switching based on information input from the touch panel 50 by an operator's operation in addition to information transmitted accompanying an image signal.

  The combination of the image forming mechanism 10, the scanning control unit 31, the mechanism control unit 32, and the image position adjusting unit 35 of the present embodiment corresponds to an example of the image forming unit referred to in the present invention. The combination of the paper transport mechanism 20 including the shift motor 256S and the transport control unit 33 including the side shift control unit 34 corresponds to an example of the paper transport unit referred to in the present invention.

  FIG. 5 is a diagram illustrating an example of signal waveforms output from the first sensor and the second sensor.

The signal waveforms output from the first sensor 257 and the second sensor 258 are composed of pulse trains corresponding to the respective light receiving elements arranged in a row. A pulse with a relatively low level means that there is no paper at the position of the corresponding light receiving element, and conversely, a pulse with a high level means that there is paper. Here, when the entire waveform is viewed, an accurate side edge position is detected by using the pulse immediately before the level of the pulse increases to a substantially maximum value as a detection reference for the side edge position of the paper. . For example, in the waveform shown in FIG. 5, a position corresponding to X ₁ is detected as a side edge position of the paper.

  FIG. 6 is a diagram illustrating a state in which an image is formed on a sheet by the image forming apparatus illustrated in FIG.

  In FIG. 6, the registration roll 256, the first sensor 257, the second sensor 258, and the photosensitive drum 12 are shown together with the sheet P on which an image is formed. FIG. 6 shows the state in which the electrostatic latent image formed on the photosensitive drum 12 is transferred to the paper P as a toner image in order from part (a) to part (e). Note that the sheet is actually curved in the skew removing unit 25 (see FIG. 3), but in FIG. 6, it is shown in a state where the whole is horizontal for the sake of easy viewing.

  As shown in part (a) of FIG. 6, the sheet P conveyed toward the registration roll 256 comes into contact with the guide member and the conveyance roll in the middle of conveyance, and the position in the width direction is changed from the standard position. It is shifted and skewed.

The first sensor 257 detects a side edge position X ₁ of the paper P conveyed toward the registration roll 256. Image position adjusting portion 35 (see FIG. 4), the first sensor 257 to calculate the latent image writing position M + X ₁ by adding a predetermined margin width M to the side edge position X ₁ has been detected.

Next, as shown in part (b) of FIG. 6, the scanning control unit 31 (see FIG. 4) causes the scanner 11 (see FIG. 1) to expose the surface of the photosensitive drum 12 with exposure light, thereby causing electrostatic latent Form an image. Scanning control unit 31 based on the calculation result by the image position adjustment section 35, controls the timing of the scanning of the scanner 11 to form an electrostatic latent image on the latent image forming position M + X _1. On the other hand, the paper P hits the registration roll 256, and the skew is removed by forming a loop. When the skew is removed, the side edge position of the paper P is also shifted.

Next, as shown in part (c) of FIG. 6, the registration roll 256 feeds the paper P to the transfer unit 15 (see FIG. 1). Note that the timing at which the registration roll 256 feeds the paper P to the transfer unit 15 is synchronized with the timing at which the formation of the electrostatic latent image on the photosensitive drum 12 is started by the scanning control unit 31. Is formed at an appropriate position in the transport direction Y. The second sensor 258, the registration rolls 256 after a predetermined time has elapsed from the start of conveyance of the sheet P, and measures the side edge position X ₂ of the sheet P.

Next, the side-shift control unit 34 (see FIG. 4) reads the side edge position X ₂ from the second sensor 258, the side edge position X _1, based on the difference between the side edge position X _2, registration The paper P is moved to the roll 256. Specifically, as shown in part (d) of FIG. 6, by driving the shift motor 256S, the side edge position _{X 1,} the difference _X 1 -X ₂ only registration roll 256 between the side edge position _{X 2} Move in the width direction.

As a result, as shown in part (e) of FIG. 6, the sheet P is returned to a position where the side edge position is X _1, and the side edge position X toner image whose position is adjusted based on ₁ photosensitive Transferred from the body drum 12. An image is formed at the position of the margin width M on the paper P.

As described above, in the image forming apparatus 1 according to the present embodiment, the scanner 11 and the scanning control unit 31 adjust the latent image writing position M + X ₁ on the surface of the photosensitive drum 12, and the registration roll 256 is used in the scanner 11. The position of the paper P after the start of scanning is adjusted. The side edge position of the paper P is likely to be shifted when the paper is abutted against the registration roll 256 and the skew is corrected after the scanner 11 starts scanning, but the registration roll 256 is the first sensor. since moving 257 definitive as the side edge position X ₁ of the paper P to the paper on the basis of the difference between the side edge position X ₂ of the sheet at the second sensor 258, the deviation of the position of the paper caused by the skew corrected (X ₂ -X ₁₎ is modified. Therefore, the alignment performance between the paper and the image is improved. Further, since the sheet is not waited after being transported to a position immediately before the toner image is transferred, a decrease in productivity can be suppressed. Further, if the sheet is moved at right angles to the transport direction, the sheet is easily wrinkled or the sheet is likely to be skewed. However, in the image forming apparatus 1 according to the present embodiment, the registration roll 256 is used to generate an electrostatic latent image by the scanner 11. Since it corresponds to the fine adjustment of the deviation occurring after the start of formation, the stress generated on the paper is reduced.

  FIG. 7 is a flowchart for explaining the process of forming an image on one sheet of paper in the image forming apparatus shown in FIG.

  In this process, first, the mode switching unit 36 selects one of the paper tray 40a to the paper tray 40d according to the paper type information, and determines whether the paper on which the image is formed is thick paper (step S11). ). Here, when the paper type is thick paper thicker than plain paper, the processing from step S51 is executed as the thick paper mode. On the other hand, when the paper type is plain paper, the mode switching unit 36 determines whether or not the mode is the high speed mode. It discriminate | determines (step S12). Here, when it is determined that the mode is the high speed mode, the process from step S41 is executed as the high speed mode, while when it is determined that the mode is the normal mode, the process from step S21 is executed. Here, the normal mode corresponds to an example of the first mode in the present invention. The mode switching unit 36 that performs the determination in step S11 corresponds to an example of the first switching unit according to the present invention, and the mode switching unit 36 that performs the determination in step S12 is the third switching unit according to the present invention. It corresponds to an example.

  In the normal mode, first, the transport control unit 33 causes the paper transport mechanism 20 to start transporting paper (step S21). Specifically, the pickup rolls 21a to 21d, the separating rolls 22a to 22d, and the transport roll 255 corresponding to the selected paper tray are driven to transport the paper. The conveyed sheet is abutted against the registration roll 256 to form a loop, and the skew is corrected as shown in part (b) of FIG.

  Subsequently, the first sensor 257 detects the side edge position of the paper that has been conveyed along the paper conveyance path R (step S22), and the image position adjustment unit 35 detects the latent edge position based on the detected side edge position. An image forming position is determined (step S23). Next, the scanning control unit 31 causes the scanner 11 to scan with exposure light at a timing based on the determined latent image forming position, thereby forming an electrostatic latent image on the photosensitive drum 12. Further, the mechanism control unit 32 controls the image forming mechanism 10 other than the scanner 11 to perform an image forming operation on a sheet (step S24).

  Thereafter, in synchronization with the scanner 11 forming an electrostatic latent image, the transport control unit 33 drives the rotation drive motor 256R to drive the registration roll 256 (step S25). As a result, as shown in part (c) of FIG. 6, the sheet is conveyed toward the transfer unit 15 of the image forming mechanism 10 by the registration roll 256. The second sensor 258 detects the position of the side edge of the sheet after a predetermined time has elapsed from the start of sheet conveyance by the registration roll 256 (step S26), and the side shift control unit 34 detects the first sensor 257 and the second sensor The amount of sheet deviation is calculated from the side edge position detected by the sensor 258 (step S27), and the shift motor 256S is driven based on the calculated amount of deviation to move the sheet in the width direction (step S28). ). Thereby, as shown in part (d) of FIG. 6, the sheet is returned to the side edge position detected by the first sensor 257 and reaches the transfer unit 15. In the transfer unit 15, the toner image formed at the latent image forming position based on the side edge position detected by the first sensor 257 is transferred to the sheet, and the transferred toner image is fixed on the sheet by the fixing unit 16. . Thereafter, the registration roll 256 stops rotating (step S29), and the shift motor 256S returns the registration roll 256 to the home position (step S30).

  Next, the processing in the high speed mode following step S41 will be described.

  The high speed mode corresponds to an example of the third mode according to the present invention, and the movement of the sheet in the width direction due to the movement of the registration roll 256 is stopped. The alignment between the paper and the image position is performed by adjusting the latent image forming position based on the side edge position detected by the first sensor 257. In the high-speed mode, the process from step S26 to step S28 in the normal mode and the process in step S30 are not performed, which is different from the process in the normal mode. The processing from step S41 to step S45 and the processing of step S46 are the same as the processing from step S21 to step S25 described above and the processing of step S29. However, in the high speed mode, the movement of the sheet by the registration roll 256 is stopped, so that the conveyance of the sheet to the transfer unit 15 by the registration roll 256 can be speeded up. Accordingly, the productivity of the entire image forming apparatus 1 is improved.

  Next, the processing in the thick paper mode following step S51 will be described.

Cardboard mode corresponds to an example of the second mode according to the present invention. Since the toner transfer onto the thick paper requires a longer time than in the case of the normal paper, the processing speed of the image forming mechanism 10 and the paper transport mechanism 20 is set low in the thick paper mode. In this thick paper mode, the second sensor 258 on the downstream side in the transport direction from the registration roll 256 detects the position of the paper, and the image position adjustment unit 35 adjusts the scanning start position based on the detection result. That is, the latent image forming position is adjusted based on the side edge position after skew correction. In the thick paper mode, the registration roll 256 transports the sheet toward the transfer unit 15 (step S51), and the second sensor 258 detects the side edge position of the sheet (step S53), and then the second sensor 258. Based on the side edge position of the paper detected by the above, the image position adjusting unit 35 determines the latent image forming position (step S23). Then, an electrostatic latent image is formed at the latent image forming position of the photosensitive drum 12 (step S55). In the thick paper mode, the operation speed of the image forming mechanism 10 and the paper transport mechanism 20 is low, so that a paper standby time occurs after the start of scanning until an image is formed on the paper, but the productivity of the image forming apparatus 1 is high. Has little effect on Further, since the paper is not moved in the width direction, the stress generated on the paper is further reduced as compared with the normal mode. In addition, since the adjustment of the latent image forming position is more accurate than the adjustment by moving the paper, the alignment performance is higher in the thick paper mode than in the normal mode.

  In the above-described embodiment, an example of a printer has been described. However, the image forming apparatus of the present invention is not applied only to a printer, and can be applied to, for example, a copying machine.

  In the above-described embodiment, the mode switching to the thick paper mode is performed based on the paper thickness information. However, the present invention is not limited to the paper thickness, and is used when image formation is performed at a low speed. Switching to the cardboard mode may be performed.

  In the above-described embodiment, the example of the photosensitive drum 12 has been described as the image carrier. However, the image carrier only needs to carry an image on the surface and circulate and can be formed, for example, in an endless belt shape. It may be a photoconductor.

1 is an external view of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a diagram illustrating a schematic configuration of the skew removing unit illustrated in FIG. 1 viewed from above the image forming apparatus. FIG. 6 is a diagram schematically illustrating how a sheet is conveyed to a deskew unit. FIG. 2 is a functional block diagram of the image forming apparatus illustrated in FIG. 1. It is a figure which shows an example of the signal waveform output from a 1st sensor and a 2nd sensor. FIG. 2 is a diagram illustrating a state in which an image is formed on a sheet by the image forming apparatus illustrated in FIG. 1. 3 is a flowchart for describing processing for forming an image on one sheet of paper in the image forming apparatus illustrated in FIG. 1.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 10 Image forming mechanism 11 Scanner 12 Photosensitive drum (image carrier)
DESCRIPTION OF SYMBOLS 14 Developing unit 15 Transfer unit 16 Fixing unit 20 Paper conveyance mechanism 25 Skew removal unit 31 Scan control unit 32 Mechanism control unit 33 Conveyance control unit 34 Side shift control unit 35 Image position adjustment unit 36 Mode switching unit 256 Registration roll (paper shift) Part)
256S Shift motor 256R Rotation drive motor 257 First sensor 258 Second sensor 259 Friction compensation member R Paper transport path

Claims (5)

It has an image carrier that carries an image on its surface and circulates and scans the surface of the image carrier with exposure light in a direction perpendicular to the moving direction of the image carrier. A latent image is formed, the electrostatic latent image is developed with toner to obtain a toner image, and the toner image is finally transferred and fixed onto the conveyed paper, thereby fixing the toner image on the paper. An image forming unit for forming an image,
A paper transport unit that transports paper on a paper transport path passing through the image forming unit, and is disposed upstream of the image forming unit in the paper transport direction, and shifts the paper in the width direction of the paper. A paper transport unit provided with a paper shift unit for adjusting the position of the paper in the width direction ,
A first sensor for measuring a position in the width direction of a sheet conveyed toward the sheet shift unit, which is disposed upstream of the sheet shift unit in the conveyance direction;
A second sensor that is provided between the paper shift unit and the image forming unit and that measures the position in the width direction of the paper conveyed toward the image forming unit ;
The image forming unit adjusts the latent image writing position in the width direction perpendicular to the paper transport direction on the surface of the image carrier based on the paper position measured by the first sensor. The image forming position in the width direction is adjusted with
The paper shift unit corrects a skew of the paper, and is based on a difference between the paper position measured by the first sensor and the paper position measured by the second sensor. An image forming apparatus that compensates for a shift amount of the sheet after the start of scanning by shifting the sheet in the width direction .   The paper shift unit is a registration roll that includes a pair of rolls that contact each other to form a nip region, and is a registration roll that corrects a skew of the paper by abutting the paper against the nip region. Item 2. The image forming apparatus according to Item 1.   The image forming unit adjusts the image forming position in the width direction of the image formed on the sheet based on the position of the sheet measured by the first sensor, and the sheet shift unit is configured to adjust the first sensor. And a first mode for shifting the sheet in the width direction of the sheet based on the difference in sheet position measured by the second sensor, and the shifting of the sheet by the sheet shift unit is stopped, and the image A first switching executed by the forming unit by switching between a second mode for adjusting the image forming position in the width direction of an image formed on the paper based on the paper position measured by the second sensor. The image forming apparatus according to claim 1, further comprising: an image forming unit.   The first switching unit obtains sheet thickness information indicating the thickness of the sheet, and switches between the first mode and the second mode based on the sheet thickness information. The image forming apparatus according to claim 3.   The image forming unit adjusts the image forming position in the width direction of the image formed on the sheet based on the position of the sheet measured by the first sensor, and the sheet shift unit is configured to adjust the first sensor. And a first mode for shifting the sheet in the width direction of the sheet based on the difference in sheet position measured by the second sensor, and the shifting of the sheet by the sheet shift unit is stopped, and the image A second unit that switches and executes a third mode for adjusting the image forming position in the width direction of an image formed on the sheet based on the position of the sheet measured by the first sensor; The image forming apparatus according to claim 1, further comprising a switching unit.

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